The present invention relates to a liquid crystal display(LCD) having a liquid crystal display panel(LCD panel)of a passive matrix display type, and more particularly, relates to a liquid crystal display having little display irregularity in which a plurality of scanning electrodes (rows) of a liquid crystal display panel are simultaneously driven.
As a method of driving a liquid crystal display panel of a passive matrix display type, a voltage averaging method described in "Liquid crystal display handbook" p. 395 to p. 399 is widely employed. According to the method, scanning electrodes corresponding to a row in the liquid crystal display panel are sequentially selected every one scanning period, a selective scanning voltage is applied, and all of scanning electrodes are scanned during a period of one frame. A data voltage at a level in the positive or negative direction around a non selection scan voltage as a center is applied to data electrodes corresponding to the column of the liquid crystal display panel in accordance with the value of display data. Further, alternating operation in which the polarity of the application voltage is inverted every predetermined time is also performed.
On the other hand, as another method of driving the liquid crystal display having a passive matrix liquid crystal display panel, there is a method of selectively driving a plurality of lines described in Japanese Laid-Open Patent Publication No. 6-67628. In the method, a selective scanning voltage corresponding to an orthogonal function (for example, Walsh function) every plurality of lines is sequentially applied to scanning electrodes corresponding to a row in the liquid crystal display panel. When all of the scanning electrodes are scanned in a period, called a period of one frame, the same operation is repeated. The operation is schematically shown in FIG. 2. FIG. 2 shows a case where the number of lines simultaneously selected is eight. The data voltage corresponding to the number of coincidence of the value of the orthogonal function in the selectively scanned line and the value of display data is applied to the data electrodes corresponding to a column in the liquid crystal display panel.
In the display to which the voltage averaging method is applied, since the levels of application voltages generated by a data driver and a scan driver is shifted close to a selective scanning voltage of the scan driver at the time current alternating operation, output amplitudes are equal. The value VLCD is given as follows by using the number N of scanning electrodes and a positive constant called a bias ratio. ##EQU1##
On the other hand, in a display to which the method of selecting and driving a plurality of lines is applied, output amplitudes Vg and Vf of the data driver and the scan driver are given by using the number m of lines simultaneously selected and the number N of scanning electrodes as follows. ##EQU2##
In the conventional liquid crystal display driving method, when a specific display pattern is displayed, display irregularity called shadowing occurs in the vertical and lateral directions. The shadowing in the lateral direction occurs since a dielectric constant of at the time of "on" display and that at the time of "off" display are different due to dielectric constant anisotropy of the liquid crystal cell.
Specifically, the dielectric constant of the liquid crystal cell when a voltage is applied ("on") is larger than that when a voltage is not applied ("off"). As the number of liquid crystal cells which are "on" on the scanning electrodes increases, the sum of electrostatic capacity seen from the scanning electrodes increases. Consequently, the scanning electrodes on which the number of liquid crystal cells which are "on" is large become largely weakened each time the selective scanning voltage changes and the effective value of the voltage applied to each liquid crystal cell on the scanning electrode is reduced to a value lower than a desired level. Consequently, for example, as shown in FIG. 9, when a plurality of bars having different lengths are displayed by turning on a plurality of cells on the background where the cells are "off", the effective value of a voltage applied to the liquid crystal cell (difference voltage of a voltage applied to the scanning electrode and a voltage applied to the data electrode) is reduced in the row in which the bar is displayed as compared with a row in which a bar is not displayed. The longer the bar display is, the more the effective value is reduced.
With respect to shadowing in the vertical direction, waveform distortion due to change in the data voltage differs according to display patterns and the effective value of the application voltage in a certain period for determining display is different every column, so that display luminance difference (display irregularity) occurs.